1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory using a charge-accumulation insulating film formed by introducing a high-valence substance into a high-dielectric material made of an oxide.
2. Description of the Related Art
In general, a flash memory that is one of non-volatile semiconductor memories is a non-volatile memory that does not require an electrical holding operation (holding power supply) for accumulation, and it is often used in electronic devices since a program and others can be readily written even after completion of a product.
In a NAND flash memory in the next and subsequent generations requiring miniaturization and a low-voltage operation, realization of a flat cell is demanded because of a difficulty in embedding inter-poly-dielectrics (IPD) in a space between cells. In order to realize suppressing an interference between cells and reducing a voltage as well as assuring an electric charge quantity based on a decrease in a contact area of the IPD and a floating gate (FG), a technology of reducing a film thickness of each of the IPD, the FG, and a tunnel film.
However, in an FG structure, a thickness of the tunnel film must be increased to a give value or above to maintain non-volatile, which stands in the way of miniaturization. In case of an FG structure in which electric charges are accumulated in a metal gate film, when a local defect occurs in the tunnel film, a great part of accumulated charges is lost through this defect. One of methods that solve a problem that the tunnel film cannot be reduced in thickness is adoption of a memory cell structure including a discrete charge accumulation layer in place of the FG structure, and a metal-oxide-nitride-oxide-semiconductor (MONOS) is a candidate for this method.
A conventional memory cell including a charge accumulation layer based on the MONOS has a multilayer structure in which a tunnel layer formed of an insulating film (an oxide film), a trap layer formed of a silicon nitride layer, a block layer formed of an insulating film, and a metal gate electrode are formed on a channel region of, e.g., a silicon substrate having a source and a drain formed therein.
Although the above-explained MONOS has a structure in which electric charges are accumulated in a silicon nitride (SiN), an amount of charge accumulation is insufficient, and there is a problem that a large threshold voltage variation range cannot be assured. When an ingenuity, e.g., increasing an amount of silicon is exercised to increase the amount of charge accumulation, a metallic state is obtained, and the same problem as that in the conventional FG structure occurs.
When a silicon nitride is used for the charge accumulation layer, a sufficient driving voltage must be applied to a tunnel film part, thereby making it difficult to realize a low-voltage operation. Further, a technology of excessively taking out electrons when erasing data to assure a sufficient threshold variation range cannot be applied to the charge accumulation layer of the silicon nitride. That is mainly because a large energy is required to further take out electrons after a state with no writing operation is achieved based on electrons. Furthermore, in the silicon nitride film, a charge trapping efficiency is poor, and threshold controllability is bad. That is because a dielectric constant is low and hence a charge trapping cross-section area is small.
Moreover, there is also an attempt to expose, e.g., TiO2 to a plasma damage in place of the silicon nitride, thereby creating a charge accumulation film. In this case, electric charges are accumulated based on occurrence of a large quantity of oxygen defects, but oxygen defects in an ionic oxide have general properties of producing a state close to a conduction band bottom. Therefore, when TiO2 or HfO2 as an ionic oxide is damaged to produce a charge accumulation layer, the accumulation layer behaves like an n-type semiconductor, and electric charges are lost due to a local defect in a tunnel film. That is, the charge accumulation layer using oxygen defects in, e.g., TiO2 has a structure that is theoretically weak in relation to electric charge holding.
For example, JP-A 2004-336044 (KOKAI) proposes a solution of this problem. JP-A 2004-336044 (KOKAI) discloses a technology of introducing an lanthanoid element into HfO2, ZrO2, or TiO2 as a charge accumulation layer. For example, in addition of La (other lanthanoid substances can be likewise added), La is a trivalent additive, a substance of +trivalence is introduced in place of +tetravalence, and oxygen becomes deficient for electric charge compensation, thereby realizing stability. In this oxygen defect, electrons cannot be stored, and an n-type-like behavior is observed. Therefore, many electric charges cannot be accumulated, and density growth of electric charges based on dopant introduction is hard to be realized.